This invention relates to the field of optical scanners such as bar code scanners. More specifically, the invention relates to the use of tunable optical components for noise reduction within optical scanners.
Electro-optical scanners, such as bar code symbol scanners, are now quite common. Typically, a bar code symbol comprises one or more rows of light and dark regions, typically in the form of rectangles. The widths of the dark regions, i.e., the bars, and/or the widths of the light regions, i.e., the spaces between the bars, when partitioned into groups, indicate encoded information to be read.
A bar code symbol reader illuminates the symbol and senses light reflected from the coded regions to detect the widths and spacings of the coded regions and derive the encoded information. Bar code reading type data input systems improve the efficiency and accuracy of data input for a wide variety of applications. The ease of data input in such systems facilitates more frequent and detailed data input, for example to provide efficient inventories, tracking of work in progress, etc.
A variety of scanning systems are known. One particularly advantageous type of reader is an optical scanner which scans a beam of light, such as a laser beam, across the symbols. Laser scanner systems and components of the type exemplified by U.S. Pat. Nos. 4,387,297 and 4,760,248xe2x80x94which are owned by the assignee of the instant invention and are incorporated by reference hereinxe2x80x94have generally been designed to read indicia having parts of different light reflectivity, i.e., bar code symbols, particularly of the Universal Product Code (UPC) type, at a certain working range or reading distance from a hand-held or stationary scanner.
FIG. 1 illustrates an example of a prior art bar code symbol reader 10 implemented as a gun shaped device, having a pistol-grip type of handle 53. A lightweight plastic housing 55 contains a light source 46, a detector 58, optics 57, signal processing circuitry 63, a programmed microprocessor 40, and a power source or battery 62. An exit window 56 at the front end of the housing 55 allows an outgoing light beam 51 to exit and an incoming reflected light 52 to enter. A user aims the reader at a bar code symbol 70 from a position in which the reader 10 is spaced from the symbol, i.e., not touching the symbol or moving across the symbol.
As further depicted in FIG. 1, the optics may include a suitable lens 57 (or multiple lens system) to focus the scanned beam into a scanning spot at an appropriate reference plane. The light source 46, such as a semiconductor laser diode, introduces a light beam into an optical axis of the lens 57, and other lenses or beam shaping structures as needed. The beam is reflected from an oscillating mirror 59 which is coupled to a scanning drive motor 60 energized when a trigger 54 is manually pulled. The oscillation of the mirror 59 causes the outgoing beam 51 to scan back and forth in a desired pattern.
A variety of mirror and motor configurations can be used to move the beam in a desired scanning pattern. For example, U.S. Pat. No. 4,251,798 discloses a rotating polygon having a planar mirror at each side, each mirror tracing a scan line across the symbol. U.S. Pat. Nos. 4,387,297 and 4,409,470 both employ a planar mirror which is repetitively and reciprocally driven in alternate circumferential directions about a drive shaft on which the mirror is mounted. U.S. Pat. No. 4,816,660 discloses a multi-mirror construction composed of a generally concave mirror portion and a generally planar mirror portion. The multi-mirror construction is repetitively reciprocally driven in alternative circumferential directions about a drive shaft on which the multi-mirror construction is mounted.
The light 52 reflected back by the symbol 70 passes back through the window 56 for transmission to the detector 58. In the exemplary reader shown in FIG. 1, the reflected light reflects off of mirror 59, passes through an optical filter 47 and impinges on the light sensitive detector 58. The filter is typically designed to have a band-pass characteristics in order to pass the reflected (return) laser light and block the light coming from other light sources. The detector 58 produces an analog signal proportional to the intensity of the reflected light 52.
The signal processing circuitry includes a digitizer 63 mounted on a printed circuit board 61. The digitizer processes the analog signal from detector 58 to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and a threshold value set by the digitizer determines what points of the analog signal represent bar edges. The pulse signal from the digitizer 63 is applied to a decoder, typically incorporated in the programmed microprocessor 40 which will also have associated program memory and random access data memory. The microprocessor decoder 40 first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyzes the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard to which the scanned symbol conforms. This recognition of the standard is typically referred to as autodiscrimination.
To scan the symbol 70, the user aims the bar code reader 10 and operates movable trigger switch 54 to activate the light source 46, the scanning motor 60 and the signal processing circuitry. If the scanning light beam 51 is visible, the operator can see a scan pattern on the surface on which the symbol appears and adjust aiming of the reader 10 accordingly. If the light beam 51 produced by the source 46 is marginally visible, an aiming light maybe included. The aiming light, if needed, produces a visible-light spot which may be fixed, or scanned just like the laser beam 51. The user employs this visible light to aim the reader at the symbol before pulling the trigger.
The reader 10 may also function as a portable data collection terminal. If so, the reader would include a keyboard 48 and a display 49, such as described in the previously noted U.S. Pat. No. 4,409,470.
In electro-optical scanners of the type discussed above, the laser source, the optics, the mirror structure, the drive to oscillate the mirror structure, the photodetector, and the associated signal processing and decoding circuitry can all be packaged in a xe2x80x9cscanning modulexe2x80x9d, which in turn is placed into the scanner""s, or terminal""s housing.
One of the factors which can affect scanner""s performance is it""s signal-to-noise ratio. The signal-to-noise ratio can be separated into two components, optical and electrical. The optical signal-to-noise ratio depends upon the amount of the reflected diffused laser light detected by the sensor 58, verses the amount of parasitic light impinging upon the sensor 58 which comes from other light sources such as an ambient light. In order to increase the optical signal-to-noise ratio, the band-pass filter is designed to transmit in the spectrum of the laser source and to block the light in other optical spectrums. A typical bandwidth of the optical filter used in bar code scanners is approximately 70 nanometers. In comparison, a typical bandwidth of a laser beam generated by a laser diode is on the order of few nanometers. The large difference between the filter bandwidth and the laser beam bandwidth is due to a number of variables.
The optical filter has to accommodate laser wavelength variations, as well as the laser wavelength changes due to temperature. In addition sufficient bandwidth must be allocated for filter variations and filter slope roll-off.
Therefore, a need exists for a scanner having very narrow optical band-pass filter. This would increase the scanner""s signal-to-noise ratio and improve the system""s performance in high optical noise environments.
The need also exists for a method of making a scanner having a narrow optical band-pass filter which would accommodate laser wavelength variations.
There is also a need for a scanner having a narrow band-pass optical filter which is capable of adjusting its filtering characteristics in order to track the laser wavelengths variations.
There is a further need for a scanner capable of controlling laser wavelengths, thus allowing the use of a non-tunable or partly tunable narrow band-pass optical filter.
Solutions for the above objects of the invention are fully disclosed in the drawings and the following pages of the specification.
These and other objects of the present invention are achieved in accordance with the present invention by a barcode reader and a method for reading barcodes in accordance with the present invention.
The barcode reader in one embodiment comprises a light source for generating a barcode illuminating beam and a detector for detecting return light reflected from a barcode illuminated by the illuminating beam and producing a signal corresponding thereto. A tunable optical filter is disposed before the detector to filter the return light. The tunable optical filter has an adjustable filtering characteristic that is adjustable to maximize the signal-to-noise ratio of the detector and thereby the barcode reader.
The light source is preferably a monochromatic light source, and most preferably, a laser light source. The tunable optical filter can be an interference type optical filter and preferably has a light collimator located in front of the filter to provide a collimated return light to the optical filter.
The filtering characteristic of the filter can be adjusted by changing an angle of incidence of the return light with respect to the optical filter. The optical filter is most preferably a band-pass tunable optical filter with a band-pass filtering characteristic, and in a preferred embodiment, the interference type optical filter includes interference coating layers preferably having a plurality of coating layers on one side of the optical filter.
In a preferred embodiment, the plurality of interference coating layers is separated into sets of layers, and one set of layers is located on one side of the optical filter, and another set of layers is located on the other side of the optical filter.
In one embodiment, the angle of incidence of return light is changed by an adjuster, which can be either passive or active. The passive adjuster is preferably a temperature sensitive bimetallic spring, and a preferred embodiment of an active adjuster is a sensor for sensing the temperature of the light source and an actuator for positioning the optical filter at an angle that increases the signal-to-noise ratio of the barcode reader.
The active adjuster can further include a lookup table for correlating a set of angular positions of the optical filter to a set of temperatures for the light source. The active adjuster can be an analog electromechanical adjuster.
In another embodiment, the active adjuster can sense the wavelength of the laser beam, and an actuator positions the optical filter at an angle that increases the signal-to-noise ratio based on the sensed wavelength. This can also be carried out by using a lookup table for correlating a set of angular positions to a set of wavelengths.
The tunable optical filter can also be an LCD type optical filter, an acoustic type optical filter, or a diffractive type optical filter.
In another embodiment of the present invention, in addition to the light source, detector and an optical filter, the reader has a controller for adjusting the wavelength of the illuminating beam to maximize the signal-to-noise ratio of the signal produced by the detector. In this embodiment, the optical filter has a static characteristic, whereas the wavelength of the light source is modulated by varying the temperature or the current to the light source. Alternatively, both the current and the temperature to the light source can be varied to adjust the wavelength of the beam.
The present invention also relates to methods for reading barcodes which include the steps of adjusting the filtering characteristic of a tunable filter to maximize signal-to-noise ratio, adjusting the filtering characteristic of the optical filter by changing the angle of incidence of the return light in order to increase the signal ratio, or controlling the wavelength of the illuminating beam to maximize the signal-to-noise ratio of the reader.
These and other features and advantages of the present invention will become more apparent from the detailed description of the invention taken with the attached drawings, wherein: